Now we finally know why the Ice Climbers aren’t on the new Smash Brothers ’ roster. According to Masahiro Sakurai, the 3DS couldn’t handle the Ice Climbers in gameplay. Our guess is that rendering two completely separate models, acting independently of one another, might have taxed the 3DS for 4 player free for alls.
Sakurai said that the Ice Climber’s relative lack of popularity (as a Nintendo Franchise, not as a Smash Character) added to Nintendo’s decision to eventually scrap the character. Of course, this seems a bit weird considering that Mr. Game and Watch and R.O.B. are both characters in the game. Sakurai also mentioned that the duo was up and running on the Wii U version of the game, but Nintendo had a desire to keep the roster static across both versions. This doesn’t mean that they won’t necessarily come out as DLC, considering Nintendo would be kind of foolish to let a complete version of the Ice Climbers though, but I wouldn’t hold your breath.
Source: Nintendo Everything